It is known in the art to utilize positive displacement superchargers having coactive rotors for supercharging internal combustion engines and for providing compressed air for other purposes. Such a supercharger may include a housing with a rotor cavity in which a pair of correlated parallel rotors pump air drawn into one end of the housing and discharged through an opening in the cavity wall near an opposite end of the housing. The rotors may be belt driven by the engine through a pulley connected directly, or through a gear train, to the pair of rotors.
Superchargers are very sensitive to running clearances. Tighter clearances between the two rotors and between the rotors and the rotor cavity wall improve flow and efficiency by reducing leakage of hot boost air to the intake side. Problems with scuffing limit the tightest practical operating clearances that are possible. Scuffing is more likely to occur during continuous high load operation from thermal growth resulting from high outlet air temperatures. Rotor fits generally compromise efficiency to maintain clearance and prevent scuffing.
Roots type and screw type compressors used on original equipment automotive engines may be made with a bearing housing between the rotors and the supercharger gear case. This bearing housing is made of aluminum on some commercial superchargers. The aluminum rotors have their air outlet ends rotatably mounted adjacent the bearing housing and are supported and driven through rotor drive stubs extending into the gear case The bearing housing and gear case are sealed by a front cover to form a drive assembly provided with a permanent charge of oil for lubricating the gears and bearings. The gear case has no positive cooling other than ram air in an automotive installation. Hot outlet air under high boost can lead to high gear case temperatures that negatively impact durability.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.